Let me explain it to you.
for the variables defined by var in python and js in the example, their scope rules are similar

1. variable scope is within the function that defines it (the entire python or js file can be regarded as one large function)
2. according to point 1: a variable with the same name as defined in the global appears within the function, and within the function, it overrides the global.

different points

To define a variable in the
1. js function, explicitly write var in front of the variable name (let's not discuss let and const), first, otherwise we will directly call the global
.
2. python defines a variable to write the variable name directly
In the
3. pyhton function, if you want to assign a global variable, you need to write the global variable name before the assignment.

based on the above premise knowledge, please see the explanation in the notes

Code of Python

 python  <br>a += 4 a = a + 4
  a, a
 js  a  
js  var python  
python  a 
a = 3
def f():
    global a // 
    a += 4
    print(a)
f()
The conversion from 
 python code to js code actually goes like this: 
var a = 3;
const f = () => {
    var a;
    a += 4;
    console.log(a) // NaN js
}